Borophosphosilicate glass (BPSG) films are widely used in microelectronic devices. For example, BPSG films may be used as a planarization film for a microelectronic circuit which provides good step coverage for high density microelectronic devices.
In fabricating BPSG films, it is well known that the amount of flow of the BPSG film varies with film composition, flow temperature, flow time and flow atmosphere. For example, the flow temperature is lowered by 40.degree. C. when the boron concentration increases approximately by one weight percent in the BPSG film. Accordingly, in the fabrication of microelectronic devices including BPSG films, it is known to control the concentration of impurities such as boron or phosphorus in BPSG films so that the BPSG flows by the proper amount at the intended flow temperature.
In order to measure the impurity concentration in the BPSG film, the BPSG film is generally monitored using a measuring instrument which uses X-rays, infrared rays, or other techniques to monitor the fabrication process of microelectronic devices. However, the measuring instrument generally requires a reference sample for comparison purposes in order to verify that the fabricated BPSG film has the requisite impurity concentration. In order to provide uniform impurity concentrations over time during microelectronic device fabrication, the reference sample BPSG film should preferably have impurity concentrations which remain stable over time.
Unfortunately, in a conventional BPSG reference sample, concentration of impurities such as boron and phosphorus may change over time as the impurities react with internal or external moisture. FIGS. 1A and 1B graphically illustrate the decrease in boron and phosphorus concentrations, respectively, over time. As a result of these decreasing concentrations, it is difficult to maintain a predetermined impurity concentration in BPSG films which are fabricated in microelectronic devices.
Changes in impurity concentrations in BPSG films may occur as follows. Initial changes in impurity concentrations may occur as the impurities in the BPSG films are diluted with moisture which is contained in the BPSG film when the BPSG film is initially fabricated on a substrate. Moreover, after the BPSG film is deposited, the impurity concentration of the BPSG film may be diluted as a result of interaction with moisture external to the BPSG film. This may result in secondary impurity concentration changes.
In view of the above changes in impurity concentrations of reference sample BPSG films, it may be difficult to measure the impurity concentrations in BPSG films of microelectronic devices during fabrication thereof.